The present invention is directed to a stem trimmer for use with patellar prostheses which are implanted in a human patella during total knee replacement surgery. In such surgery, it is common to provide a prosthesis in which one component is fastened to the distal end of the femur which has been resected and another component is fastened to the proximal end of the tibia which has been resected so that the two components will act together in permitting the leg to bend and straighten. In performing such surgery, the patella is not normally replaced but rather resurfaced such that the interior crown portion facing the condyles is cut and reamed to form a cavity in which a patellar prosthesis is implanted. The patellar prosthesis has a crown facing inwardly to engage the patella or trochlear groove of the femoral component.
Patellar prostheses come in all shapes and sizes, however, the newest designs now commonly include a body portion having a tapered surface on the side away from the crown. One such prosthesis design is the subject matter of U.S. patent application Ser. No. 07/508,088, filed Oct. 18, 1990 by the Applicant herein. Most patellar prostheses are intended to meet a stated objective of providing a functional resurfacing of the patella while eliminating as much damage and disfigurement of the patella, resulting from the implant, as possible. Human patellas are notoriously fragile and are found in a variety of shapes and cross-sections Thus, patellar implants need to be customized and sized to the patient during the replacement surgery. It is therefore desirable to utilize a patellar implant which requires a minimal amount of cutting or removal of the patient's patella in order to securely fix the patellar implant within the patella.
There are a number of factors, not the least of which is patient size, which require that patellar implants be of a number of different sizes while generally incorporating the following features. The patellar implant will usually have a domed crown facing outwardly from the patella for engagement with the condylar or trochlear groove of a femoral prosthesis component and a body portion having a stem or central post extending outwardly from the body portion in opposition to the crown. The central post or stem is intended to be fixed within a recess cut into the patella, thus securing the implant in place. The size of the body portion and length of the stems of the patellar implants will vary according to patient patella size and cross-section. In keeping with meeting the objective of minimizing damage and disfigurement of the patella structure while providing for secure entrenchment of the implant within the patella, it is a standard procedure to customize the length of the stem to accommodate the patella cross-sectional thickness.
The stem of the patellar implant is fit within a cavity reamed from the patella during the surgical procedure. It is imperative that the post and cavity provide a snug fit to ensure that the implant remains securely fixed to the patella. Since the depth of the cavity is dependent upon the cross-sectional thickness of the patella, the post, is many times, modified or truncated to meet the particular demands of the subject patella. Particular attention is required during truncation of the stem to ensure that the end of the post is squared and perpendicularly oriented with the central axis of the implant. Problems have been encountered in the situation where there is angular deviation on the end of the post; the patellar implant many times will be prevented from proper orientation within the cavity provided in the patella.
It is an object of the present invention to provide a tool for assisting in truncating or trimming the stem or post of a patellar implant.
It is yet another object of the invention to provide a tool which is adaptable to trim or truncate the stem or post of a patellar implant to various desired lengths.
It is yet a further object of the invention to provide a tool adaptable to receive patellar prostheses or implants of various sizes to trim or truncate the stem of the implant.
Yet another object of the invention is to provide a tool adaptable to receive patellar prostheses of various sizes and to trim or truncate the stem or post of the implant to various desired lengths.
These objects and others which will become readily apparent are met by the patellar stem trimmer of the present invention.